1. Field of the Invention
The present invention relates in general to belt type continuously variable transmissions of a wheeled motor vehicle, and more particularly to the belt type continuously variable transmissions of a type that comprises a primary pulley driven by an engine, a secondary pulley driven by the primary pulley through a belt, a primary pulley rotation speed sensor for detecting a rotation speed of the primary pulley, a secondary pulley rotation speed sensor for detecting a rotation speed of the secondary pulley, and a control system by which a target speed ratio (viz., a target value represented by dividing rotation speed (Np) of the primary pulley by that (Ns) of the secondary pulley) between the maximum speed ratio (viz., a low speed side speed ratio) and the minimum speed ratio (viz., a high speed side speed ratio) is calculated based on the detected rotation speeds of the primary and secondary pulleys and by which primary and secondary pulley pressures operatively applied to the respective primary and secondary pulleys are so controlled as to put a real speed ratio actually taken by the transmission into agreement with the calculated target speed ratio.
2. Description of the Related Art
As is known, the belt type continuously variable transmissions are of a type that can continuously vary a speed ratio that is the ratio between a rotation speed of the primary pulley and that of the secondary pulley. More specifically, the speed ratio is a value represented by dividing the rotation speed (Np) of the primary pulley by that (Ns) of the secondary pulley. Usually, a control system is associated with the transmission so that the speed ratio is continuously varied in accordance with an operation condition of an associated motor vehicle.
As is described in Japanese Laid-open Patent Application (Tokkaihei) 9-203459, for sensing the operation condition of the transmission, the rotation speeds of the primary and secondary pulleys are detected and, a target speed ratio of the transmission is calculated based on the detected rotation speeds of the primary and secondary pulleys and other information on the operation condition and, primary and secondary pulley pressures operatively applied to the primary and secondary pulleys are so controlled as to put a real speed ratio actually taken by the transmission into agreement with the calculated target speed ratio.
As is known, the rotation speed sensors for the primary and secondary pulleys can not detect a rotation speed that is lower than a predetermined level. For dealing with this undesirable fact, a technique has been employed wherein the target speed ratio is so set as to return the real speed ratio to the minimum speed ratio (viz., the ratio corresponding to the lowest speed position of the transmission) prior to the time when the rotation speed is no longer detected by the rotation speed sensor, and at the same time, the primary pulley pressure and the secondary pulley pressure are controlled in such a manner that the real speed ratio takes the target value while avoiding undesired slippage of the belt to the primary and secondary pulleys.
However, as is known, the speed ratio control has a limit in suppressing the slippage of the belt. Thus, for example, in case wherein the vehicle is forced to stop in a short time with a rapid deceleration, it tends to occur that the real speed ratio fails to return to the minimum speed ratio until the time when the rotation speed sensor can no longer detect the rotation speed. In this case, it is unclear whether the real speed ratio has returned to the minimum speed ratio or not, which tends to bring about a so-called speed change failure.
If the speed change control is continuously carried out even after the rotation speed sensor fails to detect the rotation speed, undesired belt slippage tends to occur because the speed change condition is not detected even if the change speed shows an excessive value. Furthermore, upon restarting of the vehicle, the undesired belt slippage tends to occur because of failure of detecting the real speed ratio. Furthermore, even if the speed change control is put off or delayed until the time when the rotation speed sensor begins to detect the rotation speed of the pulley, application of an excessive torque tends to induce the undesired belt slippage.